Question

I have a table with the following columns:

|start_date  |TZ      | 
|Dec 2, 2012 |Eastern |
|Dec 2, 2012 |GMT     |

Note 1: our server is in UTC time.

Note 2:The column start_date is a date field, not a timestamp field. Dec 2nd 2012 implicitly means “2012-12-02 00:00:00”

Note 3: The above table is actually multiple normalized tables, but for simplicity, I de-normalized it.

Note 4: I can put anything into the TZ table to make this easy.

I would like to select from my_table where start_date <= now()

However, this doesn’t work because of timezone. If the current date/time is
Dec 1st Eastern at 9PM (which is Dec 2nd 1AM UTC), the above query will return both results,
but I really only want the 2nd one. This is further complicated by daylight savings.

Ideally, I would like a query that does the following:

select * from my_table where convert_to_utc_timestamp(start_date,tz) <= now()

The above method would convert start_date to a timestamp and then convert it to the right timezone.

How would I do this in SQL?

Answer 1

There are two functions you’ll probably find useful.

The first is:

STR_TO_DATE(start_date,'%M %d,%Y')

That will get your string, in the specified format, converted to a MySQL DATE datatype.
If you have the mysql.time_zone_name et al. tables populated, you can use the function:

CONVERT_TZ()

(need to check that CONVERT_TZ takes a DATE and will return a DATETIME or TIMESTAMP, or include a time component in the string being converted to get a DATETIME, e.g.

STR_TO_DATE( CONCAT(start_date,' 00:00:00'),'%M %d,%Y %T')

Wrap that expression in the CONVERT_TZ() function, e.g.

CONVERT_TZ(  datetime_expr ,'US/Eastern','GMT')

To make use of the values stored in your TZ column, those are going to need to match, or you need to come up with a way to match to, the values stored in the mysql.time_zone_name table.